a. Field of the Invention
The present invention concerns power conversion. In particular, the present invention concerns a DC--DC half bridge asymmetrical fixed frequency pulse width modulation power converter topology, which utilizes a gapped transformer with increased magnetizing current for zero-volt switching condition from no load to full load operation. The present invention ensures transformer volt-sec and centertap capacitor amp-sec balance under any load condition with less costly components than the prior art.
b. Background of the Invention
In general, various electronic power converter topologies are used for voltage or current regulation, DC to DC power conversion, DC to AC power conversion, and power conditioning. The primary design goals in any of these power converters will include: (i) compact physical size; (ii) high power efficiency; (iii) low electro-magnetic interference (or "EMI"); (iv) low radio-frequency (or "RF") interference; (v) low part count; and (vi) low production costs. By providing a low cost power converter having a compact physical size, raw DC power can be fed throughout a major electronic system and each sub-system (or printed wiring board) may be provided with its own power converter. In switching power converters, a further design consideration is stresses imposed on the power transistors. Specifically, a well designed switching power converter will minimize voltage stresses of power transistors at light line loads and will minimize peak current stresses of power transistors at maximum output power.
As shown in FIG. 3.1 of the book Abraham Pressman, Switching Power Supply Design, p. 94, McGraw-Hill (1991) (hereinafter referred to as "the Pressman text"), a conventional half-bridge power supply uses a small capacitor C.sub.b, arranged in series with the primary winding of the transformer, to avoid "flux imbalance". (See FIG. 1.) Flux imbalance may be thought of as a DC current bias in the primary. If the volt-second product across the primary when the core of the transformer is "set" (i.e., moved in one direction along its hysterisis loop (or B-H plot)) differs from the volt-second product when the core is moved in the other direction along its hysterisis loop, the core will "walk" up or down the hysterisis loop due to such a difference. This will cause the core to saturate and consequently, switching transistors will be destroyed. Thus, avoiding flux imbalance can be thought of as maintaining "transformer balance".
The capacitor C.sub.b used in the half bridge converter discussed in the Pressman text avoids DC current bias and ensures that the voltage at the Junction of the voltage divider (formed by capacitors C1 and C2) is stable.
U.S. Pat. No. 5,245,520, issued to Imbertson (hereinafter referred to as "the Imbertson patent"), discloses a asymmetrical duty cycle power converter. (See FIG. 2a.) Specifically, during conditions of low load or highly asymmetric operation, the switch Q1 may be closed for as little as 10 percent of the duty cycle, while switch Q2 is on (i.e., closed) for the remainder of the switching period. The reflected load, controlled by the inductor L.sub.o, is the same for both the first and second legs of a switching period. That is, the inductive load L.sub.o pulls a constant current. Under such conditions, the known half-bridge topologies become relatively unstable. Specifically, to maintain DC bias voltage on the input capacitors C1 and C2, the transistors Q1 and Q2 must provide balanced amp-seconds. This requirement is illustrated in FIG. 2b in which I.sub.2 (D)=I.sub.1 (1-D). When the duty cycle is less than 50 percent (i.e., D&lt;0.5), which is known as "asymmetrical" operation, the magnitude of the current I.sub.1 will not equal the magnitude of the current I.sub.2. However, as mentioned above, the inductive load L.sub.o pulls a constant current. This dilemma results in unstable operation.
The Imbertson patent eliminates this problem by providing a balance inductor L.sub.b in parallel with the transformer. A unidirectional current I.sub.b flowing through the balance inductor L.sub.b adds to the magnitude of the bridge current I.sub.2 delivered to the load during the first part of the period and diverts bridge current I.sub.1 from reaching the load during the second part of the period. The balance inductor L.sub.b must be properly sized to assure balanced time-current products. In this way, the inductive load L.sub.o pulls a constant current and DC bias voltage is maintained on the input capacitors C1 and C2.
In addition, to assure lossless switching, even at low output loads, the device discussed in the Imbertson patent adds an inductor L.sub.c in series with the transformer. This inductor L.sub.c charges and discharges the parasitic switch capacitances much like a resonant pole converter. Unfortunately, the inductors L.sub.b and L.sub.c employed in the circuit discussed in the Imbertson patent are relatively expensive components which preclude the feasible use of such power converters on each sub-system of a major electronic system.
Thus, a low cost power converter is needed. Such a power converter should operate in a stable manner (i.e., avoid flux imbalance), even under low load conditions and asymmetric operation. It is a goal of the present invention to provide such a low cost power converter.